Based on a novel strategy, the overall objective of the proposed research is to develop new, more highly efficacious and selective drug combinations for the treatment of cancer. Numerous anticancer agents are substrates for enzymes that are subject to feedback inhibition and this regulatory mechanism can limit the activation of these drugs. Our approach to this problem involves the use of compounds which can stimulate these drug activation reactions by antagonizing feedback inhibition. We have demonstrated that 5'-amino-5'-deoxythymidine (5'-dThd) can antagonize the feedback inhibition of thymidine kinase exerted by thymidine triphosphate (dTTP) and, thereby, strongly stimulate the phosphorylation of thymidine or iododeoxyuridine (IdUrd), a cytotoxic analog. In a variety of cancer cells 5'-AdThd produces a marked increase in the uptake and cytotoxicity of IdUrd. Importantly, however, in certain circumstances this approach may produce preferential stimulation of drug activation in the cancer as compared to the normal cells. A recently developed methodology for propagating normal human urothelial cells in vitro has facilitated the comparison of these cells with a series of human bladder cancer cells. The uptake and phosphorylation of IdUrd was markedly stimulated by 5'-AdThd in the neoplastic but not in the normal cells. Since this unusual preferential stimulation may provide the basis for developing more selective cancer chemotherapy, elucidation of the biochemical mechanisms which produce these differential effects is the primary aim of this research. Several alternative hypotheses which could account for this selectivity will be tested. These include: differences in the pool sizes of the feedback inhibitors; altered levels of thymidine kinase activity; changes in regulatory properties of the thymidine kinases; and interactions with other, primarily catabolic enzymes. Since 5'-AdThd potently stimulates the uptake of IdUrd into mouse P388 leukemia cells, and because in vivo testing of this chemotherapeutic approach is important, initial studies of this drug combination in BDF1 mice will also be carried out. The primary question is whether 5'-AdThd can increase the therapeutic index of IdUrd in this animal tumor.